Crystalline aluminosilicate zeolites are known to catalyse various chemical conversion processes including the carbonylation of dimethyl ether to produce methyl acetate. Such carbonylation processes may be conducted in the presence of hydrogen but are typically conducted with an excess amount of carbon monoxide such as described in, for example EP 2189215 and EP 2199272.
EP 2189215 describes processes for the production of acetic acid and/or methyl acetate products by carbonylating dimethyl ether or methanol with carbon monoxide in the presence of a bound hydrogen form mordenite catalyst and optionally hydrogen.
EP 2199272 describes processes for the carbonylation of dimethyl ether with carbon monoxide in the presence of a mordenite catalyst, hydrogen and additional methyl acetate in which the carbon monoxide is utilised in a molar excess compared to hydrogen.
In general, zeolites are prepared by a procedure which involves crystallizing the zeolite structure from aqueous synthesis mixtures comprising sources of appropriate oxides, such as silica and alumina. Structure directing agents influence the formation of channels or tunnel like structures (a microporous structure) within a zeolite and may also be included in the synthesis mixture. Structure directing agents may be inorganic or they may be organic. Structure directing agents are removed from the formed zeolites by a variety of methods. Inorganic structure directing agents are generally removed by ion-exchange methods whereas organic structure directing agents may be removed by calcining at high temperature. Zeolites produced in this manner have been found to be useful as catalysts, as described, for example in WO 2005/105720.
WO 2005/105720 describes a carbonylation process for the carbonylation of aliphatic alcohols and/or reactive derivatives thereof in the presence of a mordenite catalyst which has, in addition to aluminium and silicon, one or more gallium, boron and iron as framework elements and which catalyst is also loaded with copper, nickel, iridium, rhodium or cobalt. The preparation of gallium mordenite is described in which tetraethyl ammonium bromide is used as an organic template and which template is removed by calcining at 550° C. prior to use in the carbonylation of methanol with carbon monoxide.
U.S. Pat. No. 7,465,822 describes a process for the carbonylation of a lower alkyl ether with carbon monoxide in the presence of a zeolite catalyst. It is disclosed that in the synthesis of the zeolite, an organic structure directing agent may be included in the reaction mixture which mixture is subsequently crystallised and calcined at high temperatures.
In general it is less costly and therefore desirable to commercially manufacture zeolites without the use of organic structure directing agents. However, an important aspect of any catalytic process is the activity of a catalyst when exposed to the desired process conditions. The improvement of catalytic performance in carbonylation reactions is a continuous objective of process and catalyst development research.
Mixtures of carbon monoxide and hydrogen (generally referred to as synthesis gas) are produced commercially and are readily available. Typically, synthesis gas mixtures are hydrogen-rich, that is hydrogen is present in such mixtures in at least an equimolar and generally in an excess molar ratio to carbon monoxide. In carbonylation processes the use of such hydrogen-rich feeds results in less space for carbon monoxide in the reactor, leading to reduced carbon monoxide partial pressures and reduced rates of reaction. Consequently, synthesis gas mixtures are processed to separate out the components carbon monoxide and hydrogen, for example by expensive cryogenic techniques. However, to avoid such costly separation of carbon monoxide from hydrogen it would be advantageous to be able to utilise synthesis gas mixtures in zeolite catalysed carbonylation processes without the need to reduce the hydrogen:carbon monoxide molar ratio thereof. Thus, a problem which exists in zeolite catalysed carbonylation processes is that in order to operate such carbonylation processes under hydrogen-rich conditions, and in particular under hydrogen-rich conditions throughout the process, necessitates an increase in the activity requirements of a zeolite catalyst.